Regional Climate: Current Research
Early Winter Pacific Northwest Precipitation Forecast Skill
- Todd Mitchell, UW Climate Impacts Group
- Nathan Mantua, UW Climate Impacts Group
- Dennis Lettenmaier, UW Climate Impacts Group
As part of a larger project to characterize the skill of hydrological forecasts for regions around the globe, the skill of two-week tropospheric geopotential height (500 hPa) forecasts is evaluated for the Northern Hemisphere extratropics during October-November-December (OND), the calendar months of floods in western Washington. The goal of this work is to document the skill of forecasts of the atmospheric component of the atmosphere-hydrology system, to understand the source of the skill (persistence or the Madden and Julian Oscillation (MJO)), and to evaluate the degree to which ENSO’s influence on the large-scale circulation modulates the skill.
The present work has been to replicate and update published skill calculations with geopotential height as both the predictand (lead) and predictor (lag) fields for December-January-February, and to repeat the calculation for October-November-December (OND). The skill of both the mechanistic NOAA Reforecast model and an empirical model constructed by canonical correlation analysis are evaluated. The OND skill has been calculated for cold, neutral, and warm ENSO episodes, with the results suggesting that the forecasts are most skillful during warm ENSO episodes (years of diminished PNW precipitation due to ENSO). Future work will finish quantitative assessments of the skill, repeat the calculations using precipitation observations as the predictor field, and repeat the calculations using the outputs of tropical intraseasonal variability (MJO) model forecasts under development by NOAA as the predictand field. Published studies have related MJO variability to western Washington floods, and documenting the limits and opportunities of forecasts at the two-week timescale is extremely useful to planners and the public.
For more publications on climate impacts on PNW climate, please see CIG Publications.